Microprocessor-based electrical power systems accumulate considerable amounts of information concerning the electrical distribution systems to which they are connected, as well as the power equipment itself. Today's utility monitoring systems provide end-users with the capability to remotely monitor a variety of equipment via automatic monitoring devices. The spectral information is used when compensating a system to reduce harmonic content and for other troubleshooting purposes.
The typical monitoring device such as a digital power meter utilizes an analog-to-digital (A/D) converter and a microprocessor, and thus all analysis is done in the discrete time or digital domain. In order to minimize error in measurements such as power, THD, etc., it is necessary to obtain voltage and current data over an integral number of cycles. Since the system frequency varies over time, it is necessary to vary the sample rate to maintain the sampling over an integral number of cycles. The input signals (such as current or voltage) are digitized by the A/D converter operating at a sampling rate which is controlled by a digital clock which may operate at a fixed or variable frequency. If it operates at a fixed frequency, it is necessary to re-sample the data in firmware to maintain the integral number of cycles in the measurement data. Regardless of which method is used, an accurate measurement of the system frequency is required. The changing of the frequency of a measured signal necessitates constant monitoring to insure accurate measurements are taken of the signal being monitored.
One known method of determining frequency is by using an algorithm that takes samples from four points in a waveform cycle and a fifth point from the previous cycle. This algorithm (the “Devaney” algorithm) is explained in “A New Quadratic Form Based Frequency Measurement Algorithm” IEEE Instrumentation and Measurement Technology Conference, Jun. 4-6, 1996. The Devaney algorithm determines the frequency of a measured signal using quadratic forms of sampled signals and does not require a fixed sampling frequency. The algorithm requires a limited number of samples and is relatively insensitive to harmonic distortion. The Devaney algorithm requires constant monitoring of each cycle sampled in order to determine frequency based on a limited number of sample points.
Thus, it would be useful to have an accurate sampling protocol that provides determination of a frequency of a waveform with stable frequency measurements.